1. Field of the Invention
The present invention relates to an electrically driven cooling cycle that makes use of the symbiotic effects of adsorption and thermoelectric cooling cycles to produce a useful cooling effect at an evaporator.
2. Description of Background Art
A central challenge in cooling science today is the development of miniaturized chillers, in particular for microelectronic appliances such as personal computers. The general aim is to develop a device that is: (1) compact; (2) virtually free of moving parts and reliable; (3) efficient in converting input to cooling power (i.e., a high Coefficient Of Performance or COP for short); (4) capable of high cooling densities typically measured in Watt per square centimeter (W/cm2); and (5) affordable. (COP is defined as the ratio of useful cooling power to input power).
Various types of cooling devices have been, hitherto, proposed or commercialised for the above-mentioned purposes. The simplest is forced air convection with the option of an extended heat sink that effectively increases the heat source surface area for heat exchange and/or the possibility of introducing ribs or barriers on the surfaces to be cooled to increase air turbulence so as to realize better heat dissipation. This method is adequate for many types of current microelectronic cooling applications; however, the current methods might cease to satisfy the compactness constraints of future generations of microelectronic cooling applications that will require a cooling density at least an order of magnitude higher than presently required.
Thermoelectric chillers are also in use, but suffer from inherently low COP (typically in the range of 0.1-0.5 for the temperature ranges characteristic of many microelectronic applications) and high cost. The low COP means that major increases in cooling density will require unacceptably high levels of electrical power input and rates of heat rejection to the environment that will be difficult to satisfy in a compact package, all at increased cost.
Passive thermo-syphons have been proposed. These devices involve virtually no moving parts, except with the possibility of one or more cooling fans at the condenser. Such a device; however, is highly orientation dependent, since it relies on gravity to feed condensate from a condenser located at a higher elevation so as to provide liquid flush back to the evaporator, which is located at a lower elevation.
Thermo-syphons equipped with one or more mini pumps have also been proposed [1]. Instead of relying on gravity, condensate is pumped from the condenser back to the evaporator. This scheme is orientation independent and also allows for the possibilities of forced convective boiling, spraying of condensate or jet-impingement of condensate at the evaporator, which will effectively enhance boiling characteristics and therefore cooling performance.
Laid-out heat pipes [2,3] have found applications especially in laptop computers. The evaporating ends of the heat pipes are judiciously arranged over the CPU while the condensing ends of the same are laid out so as to effectively increase the surface area of the heat sink.
Mini vapour compression chillers [4] have also found applications. In one design, the evaporator is arranged over the heat source surface while the mini condensing unit is positioned away from the heat source. The advantage of such a system lies in its higher COP. However, many moving parts are involved in the compressor and they have to be highly reliable. Further scaling down of the compressor for miniaturized cooling applications may also be a technical challenge, and this may lead to a sizable loss of compressor efficiency due to high flow leakages and in turn the low chiller COP.
Thermoelectric chillers [5] satisfy the requirement of compactness, the absence of moving parts except for the possibility of one or more cooling fans, and an insensitivity to scale (since energy transfers derive from electron flows). Typically, commercial thermoelectric devices comprise semiconductors, most commonly Bismuth Telluride. The semiconductor is doped to produce an excess of electrons in one element (n-type), and a dearth of electrons in the other element (p-type). Electrical power input drives electrons through the device. At the cold end, electrons absorb heat as they move from a low energy level in the p-type semiconductor to a higher energy level in the n-type element. At the hot side, electrons pass from a high energy level in the n-type element to a lower energy level in the p-type material, and heat is rejected to a reservoir.
Thermoelectric devices have found niche applications for small-scale cooling. When substantial temperature differences are needed, thermoelectric devices inherently suffer from low COP, with the concomitant drawbacks of relatively high power input, accommodating even greater heat rejection, and an appreciable cost per watt of cooling power.
Adsorption chillers have been proposed to cool electronic devices in space capsules [1]. The advantage of such devices is that they are virtually free of moving parts, except for the on-off valves that separately connect the reactors to the evaporator and condenser (therefore these units are highly reliable). Adsorption chillers are also capable of being miniaturized [6], since adsorption of refrigerant into and desorption of refrigerant from the solid adsorbent are primarily surface, rather than bulk processes [7-13]. A refrigerant such as water is exothermically adsorbed, and endothermically desorbed, from the porous adsorbent, which is usually packed in a reactor having good heat transfer characteristics.
Many adsorbent-adsorbate pairs are available, such as silica gel-water, silica gel-methanol, zeolite-water, activated carbon-nitrogen, activated carbon-methanol, etc. Silica gel-water has been the preferred pair in commercial adsorption chiller development targeted for process cooling or air-conditioning owing to: (a) silica gel""s comparatively large uptake capacity for water; (b) the high latent heat of evaporation of water; (c) the relatively low temperatures for desorption; and (d) the harmless nature of the chemicals.
However the COP of commercial adsorption chiller driven by low temperature waste heat (typically less than 85xc2x0 C.) is low, typically in the range of 0.1-0.6 for typical air-conditioning and process cooling uses. The intrinsically low COP is related to: (i) small temperature differences among the reservoirs; and (ii) the batch-wise system operating characteristics.
The technology of coupling a thermoelectric device (often referred to as a Peltier device), to an adsorber and a desorber is not new [14]. It is typically applied to humidification, dehumidification, gas purification, and gas detection. Its application in an integral chiller systemxe2x80x94i.e., to produce a thermodynamic cooling cyclexe2x80x94so as to realize the above-mentioned virtues has, hitherto, not been proposed.
In one version, a thermoelectric device is connected to one reactor [15; 16; 17]. Since one junction of the thermoelectric device is able to act either as the cooling end (with the other junction concomitantly acting as a heating end) or the heating end (with the other junction concomitantly acting as a cooling end) simply by means of switching the direction of direct current, the same junction is attached to the reactor in a thermally conductive but electrically non-conductive manner. If the reactor is designated to be an adsorber or absorber, direct current will be applied through the thermoelectric device in a manner such that the junction acts as the cooling end so that the heat generated by the adsorber or absorber is removed by the thermoelectric device to the environment. Conversely, if the reactor is designated to be a desorber or generator, the direction of flow of direct current through the thermoelectric device is reversed so that the junction acts as the heating end and supplies heat to the desorber to sustain the vapor desorption or generation. Such applications are typically found in applications related to dehumidification, gas purification, gas detection, etc.
In another version that is more relevant to the present invention, the two junctions of a thermoelectric device are separately attached in a thermally conductive but electrically non-conductive manner to two reactors [14; 18]. When direct current is applied to the thermoelectric device, the reactor attached to the cold junction acts as either an adsorber or absorber, while the second reactor attached to the hot junction acts as a desorber. When the direction of flow of direct current through the thermoelectric device is reversed, the original cold junction is switched into a hot junction, which in turn also switches the reactor from an adsorber or absorber to a desorber. Concomitantly, the original hot junction is switched into a cold junction, which in turn also switches the reactor from a desorber to an adsorber or absorber. Such applications are typically found in gas purification.
We will now explain how a unique union of the adsorption and thermoelectric chillers (electro-adsorption chiller) can produce a device that simultaneously fulfils the following aims: (a) scale independence, and hence the option of chiller miniaturization and system compactness; (b) no moving parts; (c) option of no coolant loops; (d) relatively high COP; (e) sizable cooling densities; (f) production from existing technologies (namely, its realization is not contingent upon the development of new materials or unfamiliar components); (g) modularity, which offers the possibility of assembling macro-cooling rates (of the order of kilowatts) from many miniaturized cooling units; and (h) fabrication from non-toxic environmentally-friendly materials.
In the present invention, an adsorption chiller equipped with one or more pairs of reactors is combined with one or more thermoelectric chillers. The number of thermoelectric chillers used is equal to the number of pairs of reactors equipped in the adsorption chiller. Each thermoelectric chiller is disposed such that its two junctions are separately attached in a thermally conductive but electrically non-conductive manner to two reactors, with the two reactors being in contact in a like manner only with the thermoelectric chiller and not with other thermoelectric chillers. Hence, every pair of reactors and every thermoelectric chiller form one module in the adsorption chiller.
For compactness, the electro-adsorption chiller is arranged in a modular manner, where the pairs of reactors of the chiller are linked to a vacuum-type spool valve, operated by spring-loaded and electrically-activated piezoelectric transducers that are positioned either at one or both ends of the valve piston. The outlets from the valve chambers are connected internally (to remain hermetically sealed) via the valve housing to the condenser and evaporator, respectively.
According to one aspect of the present invention, there is provided an electro-adsorption chiller assembly comprising:
a condenser, wherein the refrigerant can be cooled by forced air convection, radiation, laid out heat pipes, by liquid coolant and/or such other means that are practiced by those skilled in the art;
an evaporator that produces useful cooling, which is connected to said condenser by means of a simple on-off pressure reducing valve operated by means of electromagnetic, pneumatic, hydraulic, solid-state or other principles so as to provide a refrigerant circuit; or an evaporator that produces useful cooling which is connected to said condenser by means of a simple on-off valve operated by means of electromagnetic, pneumatic, hydraulic, solid-state or other principles and a serially connected hermetic or semi-hermetic pump that can either spray the refrigerant onto the evaporator heat exchanger surface or distribute the refrigerant via a jet-impingement technique onto the evaporator heat exchanger surface so as to provide a refrigerant circuit with markedly enhanced evaporator boiling characteristics;
one or more pairs of reactors connected by simple on-off or spool valves operated by means of electromagnetic or piezoelectric, pneumatic, hydraulic, solid-state or other principles to both the condenser and evaporator so as to provide a refrigerant circuit such that each reactor is able to operate in adsorption and desorption modes;
one or more thermoelectric chillers with their number matched to the number of pairs of reactors so that every thermoelectric chiller is dedicated to only one pair of reactors and with each of the thermoelectric chiller""s two junctions separately connected in a thermally conductive but electrically non-conductive manner which can be achieved by such means as ceramic plates to the two reactors and connected to a DC power source that is able to perform a voltage polarity switch so that each of the junctions is able to operate as a heating end and a cooling end, and able to optionally supply varying power to every thermoelectric chiller; and
control means for controlling the process time interval, the on-off control valves, the voltage polarity of the DC power source, the power supply by the DC power source to each of the thermoelectric chillers such that one of its two junctions operate as a cooling end and the reactor attached to the cooling end is cooled down while it is isolated from both the condenser and evaporator and subsequently connected serially to the evaporator to operate as an adsorber adsorbing vapour refrigerant from the evaporator for a substantial period of time and the other junction simultaneously operating as a heating end and the reactor attached to the heating end is heated up while it is isolated from both the condenser and evaporator and subsequently connected serially to the condenser and operates as a desorber desorbing vapour refrigerant to the condenser for a substantially identical time interval, and the pump, if any, that is installed between the condenser and evaporator.
In a preferred embodiment, the refrigerant is water and the adsorbent is silica gel. However, other polar fluids such as methanol, ammonia, etc. or polar dielectric refrigerant can be used. Similarly, other adsorbents such as zeolite or activated carbon can be used.
The reactor is preferably composed of good heat exchanging material and contains a predetermined amount of adsorbent. The adsorbent could be any material, such as silica gel, that is able to adsorb refrigerant, either by physisorption and/or chemisorption, for example water vapour, ammonia, methanol, etc. at a temperature dictated by the thermoelectric device""s cold junction temperature which is preferably below ambient temperature and with its lower limit dictated by the thermodynamic properties of the refrigerant so that the adsorbing capacity of the adsorbent can be markedly increased and desorbs refrigerant at a temperature dictated by the thermoelectric device""s hot junction temperature which is typically less than 100xc2x0 C.
Several known options are available for the evaporator design, depending on the cooling power density (in Watts per unit area) required and on the need for orientation independence.
In one aspect, the evaporator can be designed for a common pool-boiling mode as is typically found in conventional air-conditioning and refrigeration systems. In such a design, it would suffice by simply connecting the condenser and evaporator with an on-off pressure reducing control valve with the additional option of installing a typical flooded U-tube bend so as to maintain the pressure difference between the condenser and evaporator. However, this design can satisfy neither the orientation-independent constraint nor the requirement of cooling densities of 10 W/cm2 or higher.
In a second aspect, the evaporator can again be designed in the common pool-boiling mode with the additional installation of micro channels or even stacked-up micro channels with a honeycomb-like configuration in the pool of refrigerant and are in good thermal contact and possibly good mechanical contact with the surfaces through which thermal power is received by the pool of refrigerant so as to increase the cooling densities by effectively increasing the ratio of heat transfer surface area to volume. However, this design again cannot satisfy the orientation-independent constraint.
In a third aspect, refrigerant is mechanically sprayed onto the heat transfer surfaces of the evaporator via a distributor with the heat transfer surfaces being preferably installed with micro channels which are in good thermal and mechanical contact with the heat transfer surface of the evaporator. In this design, an on-off control valve and a downstream hermetic or semi-hermetic pump have to be installed between the condenser and evaporator so that the condensed refrigerant is pressurized by the pump and delivered to the distributor. This design is able to satisfy the orientation-independent constraint as well as the need for high cooling density.
In a fourth aspect, refrigerant is sent to the heat transfer surface by a high velocity jet-impingement method via a jet array with the heat transfer surfaces being preferably installed with micro channels which are in good thermal and mechanical contact with the heat transfer surface. In this design, an on-off control valve and a downstream hermetic or semi-hermetic pump have to be installed between the condenser and evaporator so that the condensed refrigerant is pressurized by the pump and delivered to the array of jets. This design is able to satisfy the orientation-independent constraint and the need for high cooling density. Owing to the small droplet radii produced by the injector, they could withstand a high degree of liquid superheating and thus, could remain liquid (droplet projectiles) until impingement occurs at the heat transfer surfaces.
In a preferred embodiment, the condenser is composed of a fan-cooled finned tube bundle array where the refrigerant in the tubes is cooled by forced air convection so as to realize a compact system. Alternatively the condenser could be designed with a known shell-and-tube design, where the refrigerant is contained in the shell and the coolant is driven in the tube by means of a pump and cooled by forced air convection. In yet another known design, the condenser could still be designed with a known shell-and-tube design, where the refrigerant is contained in the shell and the tubes are part of a laid-out heat pipe assembly so that heat is dissipated from the condenser through the heat pipe to the environment. An additional condenser design comprises a micro-channel heat transfer surface and/or stacked porous matrix with high thermal conductivity.
In every pair of reactors, when one reactor is being cooled and eventually functions as an adsorber, the other reactor is being concomitantly heated and eventually functions as a desorber for a substantially identical period. When more than one pair of reactors are installed in the electro-adsorption chiller, the operation of each pair of reactor is advantageously staggered so that the temperature profile in the evaporator can be smoother than that experienced when only one pair of reactors is being installed in the chiller.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.